docs: def DefineStdSpectraForUnits():
      """Define ``StdSpectrum`` attribute for all the supported
      :ref:`pysynphot-flux-units`.
  
      This is automatically done on module import. The attribute
      stores the source spectrum necessary for normalization in
      the corresponding flux unit.
  
      For ``photlam``, ``photnu``, ``flam``, ``fnu``, Jy, and mJy,
      the spectrum is flat in the respective units with flux value of 1.
  
      For counts and ``obmag``, it is flat in the unit of counts
      with flux value of :math:`1 / N`, where :math:`N` is the
      size of default wavelength set (see `~pysynphot.refs`).
  
      For ``abmag`` and ``stmag``, it is flat in the respective units
      with flux value of 0 mag. That is equivalent to
      :math:`3.63 \\times 10^{-20}` ``fnu`` and
      :math:`3.63 \\times 10^{-9}` ``flam``, respectively.
  
      For ``vegamag``, it is simply :ref:`pysynphot-vega-spec`.
  
      """
      # Linear flux-density units
      units.Flam.StdSpectrum = FlatSpectrum(1, fluxunits='flam')
      units.Fnu.StdSpectrum = FlatSpectrum(1, fluxunits='fnu')
      units.Photlam.StdSpectrum = FlatSpectrum(1, fluxunits='photlam')
      units.Photnu.StdSpectrum = FlatSpectrum(1, fluxunits='photnu')
      units.Jy.StdSpectrum = FlatSpectrum(1, fluxunits='jy')
      units.mJy.StdSpectrum = FlatSpectrum(1, fluxunits='mjy')
  
      # Non-density units
      scale = 1.0 / _default_waveset.size
      units.Counts.StdSpectrum = FlatSpectrum(1, fluxunits='counts') * scale
      units.OBMag.StdSpectrum = FlatSpectrum(1, fluxunits='counts') * scale
  
      # Magnitude flux-density units
      units.ABMag.StdSpectrum = FlatSpectrum(3.63e-20, fluxunits='fnu')
      units.STMag.StdSpectrum = FlatSpectrum(3.63e-9, fluxunits='flam')
      units.VegaMag.StdSpectrum = Vega